Climate science reached an unhappy milestone last week. And then things went a little crazy.

One of the world’s most important sentinel sites for measuring levels of carbon dioxide in the atmosphere, at Mauna Loa, Hawaii, reported that levels had recently risen above the symbolically important figure of 400 parts per million, and were likely to stay that way “for the indefinite future.”

Because rising amounts of carbon dioxide, a heat-trapping gas, in the atmosphere have been linked to climate change, scientists suggest that rising above 400 parts per million line makes it even harder to prevent global temperatures from rising beyond the goal of two degrees Celsius agreed to in Paris. It has been millions of years since atmospheric levels of CO2 have risen so high.

In recent years, monitoring stations have reported periods in which CO2 had climbed above 400 parts per million, but the new announcement from Scripps suggested that the line has probably been passed for good.

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Dr. James Hansen, a former NASA climatologist and activist, has said that CO2 levels will actually have to be reduced to 350 parts per million “if humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted.” (This is the origin of the name of the climate activism group 350.org.)

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His father, Charles David Keeling, began carbon dioxide measurements on Mauna Loa and at other locations in the late 1950s. In an interview, Ralph Keeling stressed that the 400 parts per million is “a good yardstick,” but “to call it a tipping point is incorrect.”

Gavin Schmidt, a scientist who heads a NASA climate research unit in New York, agreed. “400 ppm is definitely a milestone, but there’s no evidence it’s a tipping point,” he wrote in an email response to questions.

One danger of alarmism, Dr. Keeling said, is that it can make people feel hopeless and throw up their hands. “The first step is just to stop the increase,” he said, no matter how high the numbers rise in the meantime. “There’s no punting on global warming.”